The invention relates to a positive electrode for a secondary battery having a positive electrode active material layer containing a positive electrode active material and a positive electrode conductive agent, and to a secondary battery using the same.
Recently, portable electronic devices such as video cameras, digital still cameras, cellular phones, and lap-top computers have spread widely, resulting in a strong demand for these electronic devices with a small size, a lightweight, and a long-life. A development of a battery as a power supply, especially a small and lightweight secondary battery capable of achieving a high energy density is in progress accordingly.
In particular, expectations are very high for secondary batteries such as a lithium ion secondary battery that utilizes storage and release of lithium ions for a charge-discharge reaction, and a lithium metal secondary battery that utilizes deposition and dissolution of a lithium metal. One reason is that these make it possible to achieve a higher energy density than that achieved by a lead acid battery and a nickel-cadmium battery.
In recent years, advantages as being lightweight and high in energy density are suitable for applications in vehicles such as electric vehicles and hybrid electric vehicles, and thus research activities aiming for larger size and higher power of the secondary batteries are actively performed as well.
A secondary battery includes a positive electrode and a negative electrode together with an electrolyte. The positive electrode has a positive electrode active material layer on a positive electrode current collector. The positive electrode active material layer contains a positive electrode active material that contributes to a charge-discharge reaction.
Lithium-cobalt-based composite oxides such as a lithium cobalt oxide (LiCoO2) are widely used as the positive electrode active material. However, it has disadvantages in terms of such as price and supply. Thus, lithium-manganese-based composite oxides which are low in price and low in supply instability are used as well.
In particular, a lithium manganate (LiMn2O4), which has a spinel structure and in which an operating voltage is about 4 V on a lithium metal basis, is low in cost and excellent in safety, which is thus put into practical use gradually in applications such as an electric tool application. Further, expectations are also high for applications in vehicles.
On the other hand, a lithium-manganese-based composite oxide of a high-voltage operation type, which has a spinel structure and in which an operating voltage is 4.5 V or more on the lithium metal basis, is under review in order to realize even more higher energy density. The lithium-manganese-based composite oxide can be that which has, in addition to manganese, other transition metal element, and a general expression thereof is expressed by LiMxMn2-xO4 (where M is at least one kind of the transition metal elements other than manganese, and x is 0<x≦1). The lithium-manganese-based composite oxide makes it possible to perform the charge-discharge reaction at a higher voltage as compared to normal, resulting in the higher energy density. Further, having the spinel structure allows oxygen to be less likely to be released even at a high temperature. Thereby, it is possible to achieve both the high energy density and the high safety at the same time.
Incidentally, when an oxide having a lower conductivity than that of a metal is used as the positive electrode active material, that oxide is mixed with a positive electrode conductive agent such as a carbon material which is high in conductivity. Generally, in this case, the positive electrode active material and the positive electrode conductive agent are dispersed such as in a solvent, in addition to a binder such as a polymer material, to provide slurry, following which the resultant is coated on the positive electrode current collector to form the positive electrode active material layer.
Materials such as amorphous carbon materials and crystalline carbon materials are used as the positive electrode conductive agent, which are mixed on an as-needed basis. In a case of the amorphous carbon materials, the conductivity tends to become high since the contact area between particles increases when the specific surface area is increased. On the other hand, in a case of the crystalline carbon materials, the conductivity tends to become high when the crystallinity becomes high.
Various proposals have been made for specific examples where a carbon material is used as the positive electrode conductive agent. For example, carbon black and black lead are used in combination as the positive electrode conductive agent, in order to improve properties such as a preservation property (for example, see Patent Document 1). To improve charge-discharge cycle characteristics under a high temperature environment, a 5 V-class lithium-manganese composite oxide is used as the positive electrode active material, and acetylene black and black lead are used as the positive electrode conductive agent (for example, see Patent Document 2). To improve high temperature cycle characteristics, a lithium-manganese composite oxide having a noble potential higher than 4.4 V to a potential of a lithium metal is used as the positive electrode active material, and a carbon material in which an interplanar spacing for lattice plane (002) is between 0.344 nm and 0.352 nm both inclusive is used as the positive electrode conductive agent (for example, see Patent Document 3). Incidentally, other than the carbon material, a metal nitride or a metal oxide is also used as the positive electrode conductive agent (for example, see Patent Document 4).